Migrate metadata out of the sm_ namespace (agent_scope et al.)

[Dhravya]

Summary

Ports #92 onto the CLI-free architecture and finishes the rule behind it: nothing the plugin writes may use the sm_ prefix — that's Supermemory's internal namespace.

• Writes: agent_scope (was reserved sm_scope), agent_project_id, agent_capture_mode, agent_source.
• Reads: canonical-container profile/search requests filter on agent_scope metadata; legacy containers stay unfiltered for compatibility.
• The capture test asserts no metadata key starts with sm_.

⛔ Release gate

Do not merge/release until supermemoryai/mono#2895 (the sm_scope→agent_scope backfill) is deployed and complete. Until then, filtered canonical reads return nothing — a silent recall outage. Note for the backfill: this PR also renames sm_project_id/sm_capture_mode/sm_source; only agent_scope is read-critical, but the backfill should cover all four for consistency.

Supersedes #92, which targeted the pre-refactor file layout.

Test plan

• node --test test/unit.mjs — 23 passing; capture asserts the agent_* keys and the sm_-free invariant, recall and session-start assert the filter shape on stubbed /v4/profile requests.

🤖 Generated with Claude Code

[Dhravya]

Do not merge — dependency inventory from auditing both local repos. The sm_ keys are a cross-product contract, not plugin-internal:*

supermemory-mono reads:

• apps/api/src/lib/sm-source.ts — document source resolution from metadata.sm_source + x-sm-source header (update path preserves existing)
• apps/api/src/routes/memories/index-effect.ts — consumer-source branching (sm_source === "consumer", 2 sites) and sm_capture_mode propagation into derived memories (3 sites)
• apps/api/src/lib/posthog/index.ts — sm_capture_mode / sm_plugin_version event props
• Company Brain writes sm_source: "company-brain" (writeback.ts, company-context.ts)
• No agent_scope support on main yet — mono#2895 is still open

supermemory (console/MCP/extension):

• apps/web/hooks/use-plugin-space-meta.ts + apps/web/lib/plugin-space.ts — plugin-space rendering contract on sm_project_id / sm_source
• apps/web/lib/plugin-document.ts — document source display from sm_source
• apps/mcp/src/server/client/index.ts and apps/browser-extension/utils/api.ts — both write sm_source

Merge order for the real fix:

1. mono: accept agent_* alongside sm_* in sm-source resolution, consumer branching, capture-mode propagation, analytics (dual-read)
2. supermemory web: dual-read agent_project_id/agent_source in plugin-space + document views
3. mono#2895 backfill deployed + complete (covering all four keys)
4. merge this PR (plugin writes agent_* only)
5. later: drop sm_* dual-reads once old documents are backfilled and old plugin versions age out